The plasma membrane of an animal cell mediates the cell's interaction with its environment and regulates metabolism within the cell by controlling permeability and the activity of membrane-bound enzymes. It is clear that membrane fluidity affects all of these functions and it has been proposed that an alteration in membrane structure, specifically an alteration resulting in a change in plasma membrane fluidity, may be crucial in determining the properties of virus-transformed or cancer cells. We propose to test this hypothesis by incorporating spin label probes into the plasma membrane to report: a) the fraction of membrane lipid which is in a fluid state, and b) the viscosity of the fluid regions of the membrane. Plasma membrane will be isolated from 3T3 and SV-3T3 cells grown in tissue culture but we hope to label the plasma membrane of intact, viable cells as well. We further propose to study the mechanism by which plasma membrane fluidity is regulated in normal and transformed cells by applying spin label techniques, together with biochemical analysis of plasma membrane fractions, to cells grown under conditions which force the cell to alter cholesterol and/or unsaturated fatty acid levels.